I don't know. I do find I tend to be more inclined to read long, heavy books in winter, though my tastes in genre remain the same all year round. Could it be like with food, where you feel there are certain things you eat in summer and some you eat in winter? I know some of that is based on seasonal meat and veg, but people adhere to it quite strictly sometimes. Some books make you feel summery, whereas some make you feel warm in wintertime. But judging from the choices on that site, they're all just plain old normal books coming under this header of "Summer Reads".